The invention relates to a hydraulic chain tensioner for use with an associated chain or belt that drives an engine camshaft, balance shaft drive or the like. In particular, the hydraulic tensioner of the present invention is directed to an improvement of the construction of a hydraulic tensioner to prevent errors in the assembly of the vent device.
In general, a hydraulic tensioner includes a housing, a plunger inserted into a bore formed in the housing, and a spring that biases the plunger in the projecting or protruding direction. In the housing, a chamber is formed by assembling a hollow plunger into the bore of the housing. Oil is supplied to the chamber from an external oil pressure source. The pressurized oil acts on the plunger to cause the plunger to protrude from the housing bore and provide tension to an associated chain.
In a hydraulic tensioner, when air mixes with oil in the chamber, the plunger can be forced inwardly toward the housing due to compression of the mixed air and oil. The air/oil mixture compresses when the tension of the chain or belt increases and, as a result, the chain or belt tension can vary due to the reduced effectiveness of the tensioner.
A hydraulic tensioner, as disclosed in Japanese Laid-Open Patent No. 7-158703, discloses one proposed solution of such a problem. This hydraulic tensioner has a disc as a vent device inside the plunger. A spiral groove is formed on the disc surface. The starting end of the groove is located on the outer peripheral side of the disc surface and the terminal end is located at the center of the disc surface.
FIG. 10 shows the enlarged plunger tip portion of the hydraulic tensioner disclosed in the above-mentioned prior art patent publication. In the Figure, a hole 100a is formed at the tip of plunger 100 and disc 110 is inserted in inner chamber 100b formed inside hollow plunger 100. Spiral groove 110a is formed on one of the main surfaces (top surface) of disc 110. On a portion of the outer periphery of disc 110 a groove 110b is formed that extends in the direction of the center axis of the plunger and connects to the starting end of groove 110a. 
In such a hydraulic tensioner, air mixed into the chamber passes to spiral groove 110a from groove 110b on the outer periphery of disc 110 and leaks out to the outside air together with hydraulic oil after passing through hole 100a of plunger 100. At such time, leakage of the oil in the chamber to the outside of the tensioner is controlled by the spiral form of groove 110a on the main surface of disc 110.
Typically, product inspection after assembly of the hydraulic tensioner involves checking the ability of the chamber to retain oil pressure by introducing hydraulic oil into the chamber and measuring the rate of leakage of oil from the hole at the plunger tip.
However, in a conventional hydraulic tensioner, a disc 110 that functions as a vent device is in a single round plate shape, so that, as shown in FIG. 11, disc 110 can be assembled in an up/down reversed fashion. When such assembly error occurs, air mixed into the chamber cannot be discharged outside of the tensioner because spiral groove 110a of disc 110 is not connected to hole 110a. of plunger 100. On the other hand, when such a hydraulic tensioner, assembled in error, is inspected in the same manner as above, the hydraulic oil in the chamber can pass groove 110b on the outer periphery of disc 110, pass through the gap between the disc""s main surface and the plunger and leak out of the tensioner. The product can be judged to be acceptable, even in such case of incorrect assembly.
The objective of this invention is to offer a hydraulic tensioner that prevents incorrect assembly of the vent device and that eliminates such a conventional problem.
It is a first object of the present invention to provide a hydraulic chain tensioner including a housing with a bore formed therein. A hollow plunger is inserted slidably into the housing. The plunger has a tip with an open vent hole or aperture and a spring member that biases the plunger in the protruding direction. A vent device or disc is provided inside the hollow portion of the plunger. The vent device is composed of disc first and second portions. The first portion has a first main surface on which the vent channel is formed and the said second portion is provided on the second main surface which is on the opposite side to the first main surface. The second portion has a passage, the outer diameter of which is larger than the open vent hole in the plunger that connects to a hole in the center axis direction and extends to intersect the center axis and opens on the outer peripheral surface. The vent device is assembled into the plunger with the first main surface having a vent channel facing toward the tip of the plunger.
When the assembly is correct, fluid in the chamber passes the vent channel on the first main surface of the vent device, passes the open hole at the plunger tip and is discharged from the tensioner if fluid in the chamber is pressurized during product inspection. Air mixed in the chamber passes the vent channel on the first main surface of the vent device and leaks out to the outside air from the hole at the plunger tip.
On the other hand, when the second portion on the second main surface side of the vent device is facing the side of the plunger tip during assembly, that is, the top and bottom of the vent device are reversed in an incorrect orientation of assembly, a gap is formed between the second main surface of the vent device and the inner wall of the plunger tip because the outer diameter of the second portion of the vent device is larger than the open hole of the plunger.
When fluid is supplied to the chamber and pressure is applied inside the chamber, the fluid in the chamber flows into the passage from its opening on the outer surface of the second portion of the vent device, passes the hole in the center axis direction and is discharged in a large quantity to the outside of the tensioner.
The volume of fluid discharged from the hole at the plunger tip is large when incorrectly assembled, so that it can be separated from correctly assembled devices and, thereby, erroneous assembly of the vent device can be prevented.
It is a second object of the invention to provide a hydraulic tensioner with a vent disc having, a first portion with an outer diameter which is slightly smaller than the inner diameter of the plunger. The outer diameter of the first portion of the vent device is slightly smaller than the inner diameter of the plunger, so that a gap is formed between the outer diameter of the first portion of the vent device and the inner diameter of the plunger, and, thereby, the air mixed in the chamber passes the gap and moves to the side of the first main surface of the vent device and is discharged outside of the vent channel on the first main surface.
It is a third object of the invention to provide a hydraulic tensioner with a vent disc having a first portion with an outer diameter which is substantially equal to the inner diameter of the plunger and a groove in the center axis direction that connects to the starting end of the vent channel on the outer surface of the first portion. The outer diameter of the first portion of the vent device is substantially equal to the inner diameter of the plunger, so that when the vent device is assembled into the plunger, the outer diameter of the first portion of the vent device fits the inner diameter of the plunger without clearance. An axial groove connected to the starting end of the vent channel is formed on the outer periphery of the first portion of the vent device. Therefore, the air mixed in the chamber moves to the side of the first main surface of the vent device through the groove and is discharged outside from the vent channel on the first main surface.
It is a fourth object of the invention to provide a hydraulic tensioner which has a vent channel with a tortuous path from the starting end to the terminal end. The vent channel may be circuitous from the starting end to the terminal end so that flow of liquid leaking from the chamber is restricted and the leak volume is suppressed. Other shapes of vent channels are contemplated including a spiral-shaped vent channel or a vent channel composed of multiple linear portions that bend at least 90 degrees.
It is a fifth object of the invention to provide a hydraulic tensioner with a passage that opens on the outer surface of the second portion of the vent device extending radially from the center. When the top/bottom of the vent device are reversed during assembly into the plunger and when fluid is supplied to the chamber and pressure is applied to the chamber during product inspection, the fluid in the chamber enters into an axial hole from the radial passage and is discharged in large volume to the outside of the tensioner from the hole at the plunger tip.
It is an sixth object of the invention to provide a hydraulic tensioner with a groove-shaped passage that is formed on the end face of the second portion. The passage opening on the outer periphery of the second portion of the vent device can be a groove formed on the end face of the second portion.
It is a seventh object of the invention to provide a hydraulic tensioner with a vent device biased toward the tip side of the plunger by the force of a spring. The vent device is pushed toward the plunger tip by the force of the spring. Thereby, the first main surface of the vent device can be contacted closely with the inner wall of the plunger tip during correct assembly.
It is an eighth object of the invention to provide a hydraulic tensioner with a vent device built integrally with a pressure relief valve assembly, in which the pressure relief valve assembly is pressed toward the tip of the plunger by the said spring. The vent device has a through-hole that connects to the center hole of the second portion, at the center of the first portion of the vent device The vent device is assembled onto the pressure relief valve assembly and the pressure relief valve assembly is pushed toward the plunger tip by the spring. At the same time, a through-hole, that connects to the axial hole of the second portion, is formed at the center of the first portion of the vent device. Thereby, when the pressure on the inside of the chamber becomes high, the fluid in the chamber is discharged to the outside of the tensioner through the through-hole of the first portion and the axial hole of the second portion of the vent device. As a result, abnormally high pressure in the chamber is prevented and the integral assembly of the vent device with the pressure relief valve assembly makes the entire hydraulic tensioner compact.
It is a ninth object of the invention to provide a hydraulic tensioner with the outer diameter portion of the second portion of the vent device inserted into a hole formed on one end of the pressure relief valve assembly. The outer diameter of the second portion of the vent device is inserted into the hole formed on one end of the pressure relief valve assembly, so that the hydraulic tensioner can be made more compact.
It is a tenth object of the invention to provide a hydraulic tensioner with a passage in the housing that connects the chamber to the external pressure source. A passage to connect the chamber to the external pressurized fluid source is provided in the housing so that the fluid from the external pressurized fluid source is supplied to the chamber through this passage. A check valve is provided between the chamber and passage so that the flow of fluid to inside the chamber from the pressurized fluid source is permitted, but meanwhile, the reverse flow of fluid is blocked.
It is an eleventh object of the invention to provide a hydraulic tensioner with a check valve that permits the flow of fluid to the chamber but blocks the reverse flow of the fluid provided between the chamber and the passage.
For a more complete understanding of the invention, one should refer to the embodiments illustrated in greater detail in the drawings and described below by way of examples of the invention.